1. Field of the Invention
The present invention relates to a sealed secondary cell having a terminal pole serving also as an external terminal.
2. Prior Art
Hitherto, the majority of demands of secondary cells were occupied by small cells used as the power source of portable appliances such as VTR camera, and the technical development has been promoted mainly in the direction of smaller size and larger capacity. Recently, however, there is an increasing demand for stationary secondary cells used as emergency power source such as no-break power unit installed in a computer system, secondary cells as mobile power supply used in electric vehicle (EV) developed as environmental protection or energy-saving measure, and high performance secondary cell of medium capacity or large capacity ranging from scores to hundreds of Ah (ampere-hours). In particular, the secondary cell used in EV or other mobile use is required to satisfy the acceleration equal to the internal-combustion engine such as gasoline engine, high output characteristic and high energy density to withstand a large current load of 100 A (amperes) or more to obtain a sufficient traveling distance, and high reliability enclosed structure of a long maintenance-free period for the ease of handling. Research and development efforts continue with respect to sealed nickel-cadmium storage cells, nickel hydrogen storage cells, alkaline secondary cells, and lithium secondary cells, among others, as batteries which surpass the performance of the improved type lead storage cell hitherto considered for use in EV. To enhance the output characteristic of these batteries, however, it is desireable to improve the characteristics of positive and negative plates, while considering the optimal structural conditions of the plate group, such that the electric power obtained from the plate group is delivered to the external terminal of the battery with minimal power loss. Accordingly, it is desireable to decrease the electric resistance of the connection between plural lead plates and the external terminal of the battery and keep the resistance between the external terminal and external coupling plate as low as possible. Moreover, to meet the demand for an enclosed battery having high reliability, a joining method is required which can connect an external terminal to the battery lid with a seal which is air-tight and liquid-tight.
In conventional alkaline secondary cells or lithium secondary cells which are enclosed and have medium to large cell capacities as discussed above, typically terminal poles were employed which have the same or similar shapes as those used in vent type nickel-cadmium storage batteries having low Ah ratings (i.e., less than 100 Ah). The size, material, or design of these batteries were changed depending on the battery capacity or the like. Various examples of alkaline storage battery terminal pole structures are disclosed, for example, in Japanese Laid-open Utility Model 49-88221, Japanese Laid-open Utility Model 57-23869, and Japanese Laid-open Utility Model 61-99966. An essential lateral sectional view near terminal unit of a conventional alkaline storage battery is shown in FIG. 9.
The terminal unit shown in FIG. 9 is a screw tightened terminal.
In FIG. 9, a terminal pole 62 is formed by connecting a current collector 62c for connecting a flange 62b and plate lead plate to the lower end of a pole 62a having threads 62d, integrally by cutting, or fabricating separately and welding. The material for the terminal pole 62 is, when applied in an alkaline secondary cell, generally nickel plated steel, nickel alloy, or other corrosion resistant metal. A lid 61 is made of a synthetic resin. In a pole insert hole 6a provided in the lid 61, the pole 62d of the terminal pole 62, and two annular packings 63a, 63b are inserted, and the terminal pole 62 is tightened and fixed to the lid 61 by tightening a nut 65 on the threads 62d provided in the pole 62a through a metal washer 64. The annular packings 63a, 63b are made of synthetic rubber or synthetic resin. In this prior art, the external terminal is formed by a step 62e and a terminal screw 62f provided in the upper part of the pole 62a, and a perforated coupling plate is disposed in the step, so as to be connected by screwing. In these cell systems, the terminal poles are mostly fixed by screwing on the lid, and by the use of integrated terminal pole, the electric resistance from the plate group to the external terminal portion is lowered.
In this conventional structure, thread cutting of the nut 65 and pole 62a and tightening of nut are desireable, and the labor is complicated. To eliminate this defect, without using the nut 65 for tightening the pole, a vent type alkaline storage battery in a structure for fixing the terminal pole by using an annular spring having a central opening is proposed (Japanese Laid-open Utility Model 54-171143). An essential lateral sectional view near the terminal unit of this vent type alkaline storage battery is shown in FIG. 10 (A), and a perspective view of the annular spring composed in the terminal unit is shown in FIG. 10(B).
In FIG. 10(A) and FIG. 10(B), a terminal pole 72 is composed of a pole 72a, a step 72b of circular flange form abutting against the lower side of a lid 71, and a current collector 72c. The pole 72a projects outward through an insert hole 71a provided in the lid 71, and a rubber annular packing 73 for keeping airtight between the pole 72a and insert hole 71a is mounted on an annular step 71b of the lid 71, and a washer 74 is placed on the packing 73. A central opening 75a of an annular spring 75 is inserted into the pole 72a, and is pressed in to penetrate through. In this case, a spring upper end 75b is inserted and fixed in an annular groove 72d provided in the middle peripheral edge of the pole 72a, and the annular packing 73 held between the lid 71 and washer 74 is pressed and deformed in the radial direction, by an elastic force of the annular spring 75 against the step 72d of the terminal pole 72, and an extended portion 73a is hit against the pole peripheral edge, and the terminal pole 72 is fixed in the lid 71 while keeping airtight by plugging the gap between the pole 72a and insert hole 71a of the lid 71. In the annular spring 75, as shown in FIG. 10(B), a notch 75d is formed at the upper end (opening) side.
When using the same structure as a conventional small-sized battery of medium to large size, since the required performance and condition of use are severe, it was difficult to maintain the battery performance for a long period by keeping airtight and liquid tight for a long time.
That is, to enhance the output density and energy density of the battery, it is desirable to minimize the electric resistance and suppress the dimension in the height direction as far as possible in design, concerning the terminal pole and its sealing structure. Moreover, in particular, the following conditions for use of the battery are desireable.
(1) At the present, in a narrow space of electric vehicle or the like, about scores to 250 cells are connected in series to be used in high voltage stage. Therefore, if electrolyte leaks out from a terminal or other part of a certain cell, leak current may cause discharge, or lead to short circuit, heat generation, ignition, or electric shock.
(2) The operating ambient temperature is -30.degree. to 50.degree. C. or a wider range, and the reserved heat temperature rise of the battery due to charge and discharge of a large current in a narrow accommodation space is significant. As a result, in high temperature condition, the battery temperature may exceed 70.degree. to 100.degree. C.
(3) A battery life equal to or longer than the life of the appliance (for example, vehicle), 5 to 10 years or longer, is desireable.
The problems in the prior art are described below.
First, in the terminal pole by screw fixing shown in FIG. 9, the inventors were aware that problems occurred in the screw tightening process. That is, between the pole 62a and the pole insert hole 61a for inserting the pole 62a, there is a slight gap for the ease of processing and assembling of parts. The nut 65 for tightening must be relatively thin as compared with the screw diameter, as shown in the drawing, in order to enhance the battery energy density. To realize a battery of large size and large capacity, the pole 62a must be thick and large, and hence the screw diameter for fixing the pole increases, thereby increasing the thread pitch and play. In such constitution of parts, by rotating the nut 65, when compressing spherical packings 63a, 63b such as O-rings to a thickness of about 1/3 to 1/2, the nut 65 may be tightened at an inclination of angle .theta. as shown in FIG. 8. This problem tends to expand further in mass production. In such screw tightening state, tightening of the annular packings 63a, 63b is uneven, and the air tightness or liquid tightness of the pole may be lowered, and leakage may occur in an early phase. Besides, as mentioned above, when inserting the perforated coupling plate into the terminal screw 62f above the pole 62a to mount onto the step 62e, and tightening the nut to connect the terminal screw 62f to the coupling plate to couple between cells, in order to lower the connection resistance in the coupling portion, it is desireable to tighten the coupling plate firmly, and the nut is tightened to a torque of about 70 to 200 kgf-cm. At this time, the torque is transmitted to the whole pole through the pole 62a, and the plate group connected to the current collector 62c receives a torsional stress through plural lead plates, which may lead to short circuit between plates, or lowering of characteristic or breakage due to disconnected of welding of lead plates. On the other hand, in the method of tightening by putting the perforated coupling plate directly on the upper side of the nut 65, without forming step in the pole 62a, the nut may be rotated together to lower the air tightness, or the nut may be tightened obliquely, and the contact resistance increases. Or, in a method of tightening by using a strap nut serving also as pole and current collector, without using metal washer 64, in the state of holding the lid by two annular flat packings made of resin, the rotating nut flaws the packing. If dust deposits, this phenomenon is intensified, and anyway the tightness is lowered.
In a proposed improved vent type alkaline storage battery as shown in FIG. 10(A), simplification of labor is intended by eliminating screw tightening in fixing of pole. As mentioned above, the rubber annular packing 73 is compressed in the vertical direction by pressing force of the annular spring 75 through the washer 74, and is deformed in the radial direction, and the extended portion 73a of the packing caused by this abuts against the peripheral edge of the pole 72a so as to seal tightly. In this method, the sealing capability is determined by the pressing force and its duration of the leading end of the extended portion 73a in vertical free state abutting against the pole 72a. In other words, the sealing capability is determined by the repulsive elastic force of the rubber annular packing 73, and its compression rate, and the packing material is a dominant element. As the material for the annular packing 73, synthetic rubber or resin having rubber-like elasticity is used, but in this case, as in the above structure, when sealing at the extended portion of the packing, it was difficult to obtain a sufficient pressing force in the area by the repulsive elastic force alone. The vent type alkaline storage battery is used in an application where electrolyte supplement and cleaning are possible, and it is designed to exhaust the gas generated during overcharge through a low pressure exhaust valve or the like. In this vent type alkaline storage battery, the battery internal pressure is low, being less than 1 kgf/cm.sup.2, and hence there was no problem in sealing even in the terminal pole fixed structure as in FIG. 10(A).
In that sealed secondary cell, especially in the alkaline secondary cell, the internal pressure is usually 2 to 5 kgf/cm.sup.2 due gas generated in the battery, or reaching around 10 kgf/cm.sup.2 in a specific condition. Accordingly, in such a method of sealing by the extended portion 73a of the packing as mentioned above, the leading end of the extended portion 73a is lifted from the peripheral edge of the pole 72a by the internal pressure, and gas leaks and mist-like electrolyte leaks out onto the surface of the lid 71, and as a result the characteristic is lowered and handling troubles are likely to occur, and hence it was difficult to assure the quality. To increase the pressing force at the leading end of the extended portion 73a of the packing, there is means for increasing the compression rate of the annular packing 73. However, if the compressive rate of the annular packing 73 is large, the compressive permanent strain rate of the packing material increases, and hence creep phenomenon (plastic deformation) occurs in a short period, and the sealing performance is lowered. Besides, in the wide range of battery operating temperature, especially in summer when the battery temperature may exceed 70.degree. C., deterioration of packing material is added, and the creep phenomenon is amplified and the pressing force decreases significantly, and hence it was further difficult to maintain the sealing performance for a long period. It also may be required to decrease the dimensional tolerance of the inner and outer diameter of the annular packing 73, outer diameter of the pole 72a and diameter of the annular step 71b of the lid 71. If the dimensional tolerance is large, a runout may occur between the annular packing 73 and pole 72a, and hence uneven pressing force of the extended portion 73a of the packing takes place and the air tightness may drop.
A lateral sectional view near the terminal unit in another conventional vent type alkaline storage battery is shown in FIG. 11. The sectional view shown in FIG. 11 is a structure of fixing a terminal pole 82 to a lid 81 by using an annular spring 85, same as in the prior art shown in FIG. 10(A). Annular packings 83c, 83b are disposed on upper and lower sides of the lid 81, and the upper and lower stages abut against the peripheral edge of the pole 82a. In such a structure, leakage of electrolyte is decreased. In the vent alkaline storage battery shown in FIG. 11, as in the prior art in FIG. 10(A), the lid 81 is provided with an insert hole 81a for insert made of synthetic resin and the pole 82a, and annular types 81b, 81c are provided, as shown in the drawing, in the upper and lower corners of the insert hole 81a. The terminal pole 82 is formed by integrally assembling a columnar pole 82a having a terminal for connecting a coupling plate in the upper part, and a disk-shaped flange 82b abutting against the terminal pole 82 at the lower side of the lid 81 at the lower part, and a current collector 82c is provided at the lower end. The upper and lower annular packings 83c, 83b are rubber-made O-rings. By the elastic force of the spring occurring between the annular spring 85 and the flange 82b of the pole, the lower annular packing 83b is pressed between the flange 82b of the pole and the lower taper 81b of the lid 81, and the upper annular packing 83c is pressed between the washer 84 and the upper taper 81c. At this time, the spherical packings 83b, 83c are pressed by the tapers 81b, 81c so as to fill up the gap between the penetration hole 81a and pole peripheral edge, and deformed by receiving a stress inward in the radial direction by the tapers 81b, 81c, thereby abutting against the pole peripheral edge. Thus, when applied in the storage battery of low internal pressure such as vent type, by using double annular packings, an effect is noted in suppression of leakage of electrolyte by capillary action as compared with the prior art in FIG. 10(A). It is, however, same as in the constitution in FIG. 10(A) that the clearance in the pole peripheral edge is sealed by the elastic force (rubber elasticity) of the annular packings 83b, 83c themselves, and therefore physical properties are lowered by aging effects of packing material and high temperature deterioration, and a sufficient air tightness is not obtained at the battery internal pressure of 3 to 5 kg/cm.sup.2 or higher, and the balance of battery composition may be broken due to gas leak in a long course of use, which resulted in deterioration of battery performance due to decrease of electrolyte, and leakage of electrolyte. Owing to such reasons, it was improper to apply the terminal structure as shown in FIG. 10 into the novel sealed secondary cell. As the terminal structure of secondary cell, the structure of forming the pole and lid hole in a taper form narrow at the upper side and putting the O-ring into the taper clearance (Japanese Laid-open Patent 57-15356), and the structure of forming the lid hole and rubber packing in an outward open taper form, and tightening the packing by screw (Japanese Laid-open Utility Model 61-42776) have been proposed among others, but anyway the occupied dimension in the height direction is large, the energy density is lowered, and the same problems as in the prior art shown in FIG. 10 and FIG. 9 are present, and hence it is difficult to apply in the novel sealed secondary cell.
Moreover, in the terminal structure shown in FIG. 10(A) and FIG. 11, since the pole is fixed to the lid only by the pressing force of the annular spring, same as in the prior art in FIG. 9, when connecting the coupling plate by tightening the nut, the current collectors 72c, 82c receive a torsional stress by external pressure such as torque, and hit stress is larger than in the terminal structure of the screw tightening type in FIG. 9, and moreover the plate group may be short-circuited inside, and factors for deterioration of battery are involved, and handling of the battery is made difficult.
It is hence a primary object of the invention to solve the problems of the conventional fixing method of terminal pole, and present a sealed secondary cell of medium to large capacity, capable of exhibiting a stable battery performance for a long period in environments including high temperature.